DC Motor Question

Doesn't the controller have a signal level input that disables the the armature current? That's what you want to use. It's generally best to avoid switching the DC to an inductive load if possible.

If it's this controller, or similar, the "inhibit" input is what you want to use.

There is an inhibit method or two you can use. It's all documented in their online manuals. You interrupt the signal of the speed pot I believe. So it sees o volts = no speed. The pot is 0-10v.

Please correct me if I'm wrong Mike

You said traction motor - you did not specify locomotive. An electric car uses a traction motor. So does a forklift and even an electric bicycle if you want to stretch the point.